1. Cross-Reference to Related Application
The present application claims priority to European Application Number 99105662.3, filed on Mar. 19, 1999 by R. Allenspach et al., assigned to the assignee of the present application.
2. Technical Field
The present invention relates to magnetic devices and generally to devices having a pinning layer. More particularly the invention relates to magnetic memories (MRAM) and magnetoresistive sensors based on the so-called xe2x80x9cspin-valuexe2x80x9d or xe2x80x9cgiant magnetoresistive (GMR)xe2x80x9d effect. Although the present invention is applicable in a variety of magnetic applications it will be described with the focus put on an application to magnetoresistive sensors as GMR sensors, for example.
3. Description of the Related Art
The change in electrical resistance of a material in response to a magnetic field is called magnetoresistance which has made it possible to read information on a magnetic medium, such as a computer hard disk. The prior art discloses a magnetic read tranducer referred to as a magnetoresistive (MR) sensor or head which has been shown to be capable of reading data from a magnetic surface at great linear densities. A MR sensor detects magnetic field signals through the resistance changes of a read element fabricated of a magnetic material as a function of the strength and direction of magnetic flux being sensed by the read element. These prior art MR sensors operate on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization and the direction of sense current flow through the element. A more detailed description of the AMR effect can be found in xe2x80x9cMemory, Storage, and Related Applicationsxe2x80x9d, D. A. Thompson et al., IEEE Trans. Mag. MAG-11, p. 1039 (1975).
More recently, a different, more pronounced magnetoresistive effect has been described in which the change in resistance of a layered magnetic sensor is attributed to the spin-dependent transmission of the conduction electrons between the magnetic layers through a nonmagnetic layer and the accompanying spin-dependent scattering of electrons at the layer interfaces and within the ferromagnetic layers. This magnetoresistive effect is variously referred to as the xe2x80x9cgiant magnetoresistivexe2x80x9d (GMR) or xe2x80x9cspin valvexe2x80x9d effect. A magnetoresistive sensor based on the before-mentioned effect provides improved sensitivity and greater change in resistance than observed in sensors utilizing the AMR effect. The electrical resistance read-out means that the signal is much stronger in such GMR sensors. The increased signal offered in the GMR sensor allows more information to be stored on a hard disk. For a bit that aligns the ferromagnetic layers parallel under the GMR sensor, the resistance goes down, the electrons do not scatter very much and the current flow increases. Such a sensor can also use a multifilm laminated pinned ferromagnetic layer in place of the conventional single-layer pinned layer.
U.S. Pat. No. 4,949,039 to Grunberg describes a layered magnetic structure which yields enhanced MR effects caused by a antiparallel alignment of the magnetizations in the magnetic layers. Grunberg describes the use of antiferromagnetic-type exchange coupling to obtain the antiparallel alignment of the magnetizations in the magnetic layers. Grunberg describes the use of antiferromagnetic-type exchange coupling to obtain the antiparallel alignment in which adjacent layers of ferromagnetic materials are separated by a thin interlayer of Cr or Y.
U.S. Pat. No. 5,206,590 to Dieny et al. discloses a MR sensor in which the resistance between two uncoupled ferromagnetic layers is observed to vary as the cosine of the angle between the magnetizations of the two layers. This mechanism produces a magnetoresistance that is based on the spin valve effect and, for selected combinations of materials, is greater in magnitude than the AMR.
The U.S. Pat. No. 5,159,513 to Dieny et al. discloses a MR sensor based on the above-described effect which includes two thin layers of ferromagnetic material separated by a thin film layer of a nonmagnetic metallic material wherein at least one of the ferromagnetic layers is of a cobalt or a cobalt alloy. The magnetization of the one ferromagnetic layer is maintained perpendicular to the magnetization of the other ferromagnetic layer at zero externally applied magnetic field by exchange coupling to an antiferromagnetic layer.
Published European Patent Application EP-A-0,585,009 discloses a spin valve effect sensor in which an antiferromagnetic and an adjacent magnetically soft layer cooperate to fix or pin the magnetization of a ferromagnetic layer. The magnetically soft layer enhances the exchange coupling provided by the antiferromagnetic layer.
The spin valve structures described in the above-cited U.S. patents and European patent application require that the direction of magnetization in one of the two ferromagnetic layers be fixed or xe2x80x9cpinnedxe2x80x9d in a selected orientation such that under non-signal conditions the direction of magnetization in the other ferromagnetic layer, the xe2x80x9cfreexe2x80x9d layer, is oriented either perpendicular to (i.e. at 90xc2x0) or antiparallel to (i.e. at 180xc2x0) the direction of magnetization of the pinned layer. When an external magnetic signal is applied to the sensor, the direction of magnetization in the non-fixed or xe2x80x9cfreexe2x80x9d layer rotates with respect to the direction of magnetization in the pinned layer. The output of the sensor depends on the amount of this rotation. In order to maintain the magnetization orientation in the pinned layer, a means for fixing the direction of the magnetization is required. For example, as described in the above-cited prior art documents, an additional layer of antiferromagnetic material can be formed adjacent to the pinned ferromagnetic layer to provide an exchange coupled bias field and thus pinning. Alternatively, an adjacent magnetically hard layer can be utilized to provide hard bias for the pinned layer.
Another magnetic device is a magnetic random access memory (MRAM) which is a non-volatile memory. This memory basically includes a GMR cell, a sense line, and a word line. The MRAM employs the GMR effect to store memory states. Magnetic vectors in one or all of the layers of GMR material are switched very quickly from one direction to an opposite direction when a magnetic field is applied to the GMR cell over a certain threshold. According to the direction of the magnetic vectors in the GMR cell, states are stored, and the GMR cell maintains these states even without a magnetic field being applied. The states stored in the GMR cell can be read by passing a sense current through the cell in a sense line and sensing the difference between the resistances (GMR ration) when one or both of the magnetic vectors switch. The problem is that in most GMR cells the GMR ratio is relatively low (e.g. 10% or less) and, consequently, reading or sensing the state stored in the GMR cell can be relatively difficult.
In general, magnetic devices often use an antiferromagnetic layer to pin the magnetic moment of a subsequently deposited ferromagnetic layer. Typically used materials are FeMN, NiMn, CoO, NiO, and TbCoFe. The main advantage of using exchange bias is that the bias field cannot be reset or changed accidently during the lifetime of the device. To reset the antiferromagnet it is necessary to cool the antiferromagnet from above its Nxc3xa9el temperature in the presence of a magnetic field. A disadvantage of FeMn is that this material is a metal and allows a current flow. Therefore, FeMn is not ideally suited as pinning material. NiO is an insulator, thus having the disadvantage that the strength of the pinning, i.e. the exchange bias, is not as strong as desired.
Since the load of data which have to be stored increases dramatically, there is a need for faster operation in read and write processes with higher density. Thus, the operating temperature of the data storage systems is increasing. Today""s sensors show some drawbacks and are hence not suitable for new generations. For example, the currently used antiferromagnet in GMR sensors is NiO with a Nxc3xa9el temperature TN of about 450 K whereas the operation temperature of the head is about 400 K. There is only a small gap between these two temperatures. If the operation temperature reaches the Nxc3xa9el temperature the effect of the exchange bias and therefore the pinning of a ferromagnetic layer disappears because the antiferromagnetic material becomes paramagnetic. A further disadvantage of the prior art is that the efficiency of the exchange bias of the pinning material, i.e. the antiferromagnet, drops caused by thermal charging or electrical spikes. Hence, some spins change their direction which influence the stability of the exchange bias. Furthermore, some antiferromagnetic materials show the disadvantage of losing the orientation of their spins caused by aging. There is also the fact that commonly used metallic antiferromagnets are prone to oxidation and corrosion phenomena. Another disadvantage is that often rather thick antiferromagnetic layers of about 50 nm or more are used as the exchange bias.
Exchange bias leads to a shift of the M(H) hysteresis because of uniaxial anisotropy. The exact control of the exchange bias is a difficult materials science problem because it depends on atomic details of the interface that are, by their very nature, difficult to measure. The magnitude and sign of the exchange coupling between a pair of atoms is a rapidly varying function of the atom-to-atom spacing.
Not all materials which are stated in the literature to be antiferromagnetic in bulk conform with high Nxc3xa9el temperature work. For example, Fe3Al was reported by G. Rassman and H. Wick, Arch. Eisenhuttenw., 33,115 (1963) to be antiferromagnetic with a high Nxc3xa9el temperature of 750 K; but iron-aluminum files at and near the Fe3Nl composition were tried and did not give unidirectional anisotropy. Other alloys have been reported to have a high value of TN but when they were tried, they also did not work. They include: Alxe2x80x94Cr alloys near AlCr2 composition; MnPd alloys near MnPd composition; CrMn alloys from about 1% to 90% Mn. Neither CrMn nor MnPd has a stable gammaMn phase at room temperature.
The state of the art shows that it is rather difficult to find suitable antiferromagnetic materials with advanced properties in order to produce unidirectional anisotropy. Furthermore, practical antiferromagnetic materials with Nxc3xa9el temperatures TN greater than 450 K are desired.
Since currently used magnetic devices, e.g. as part of magnetoresistive sensor, are not ideal for novel generations with higher operating temperature, improved structures of such magnetic devices are required.
Therefore, an object of the present invention is to overcome the disadvantages of the prior art.
It is another object of the present invention to achieve a magnetoresistive structure which induces a strong pinning effect at high operating temperature.
It is still another object of the present invention to optimize the properties of the magnetoresistive structure.
It is a further object of the present invention to provide an alternative pinning layer for a magnetic device having a high Nxc3xa9el temperature TN.
It is still a further object of the present invention to allow the use of a thin pinning layer, e.g. in GMR devices.
It is also an object of the present invention to enable the manufacturing of an improved magnetoresistive structure.
The objects of the invention are achieved by the features of the enclosed claims. More specifically, the underlying concept of the present invention concerns at least an antiferromagnetic layer, which is in direct contact with a ferromagnetic layer for inducing an exchange bias in the ferromagnetic layer. Thus, the ferromagnetic layer is pinned by the antiferromagnetic layer, also referred to as the pinning layer. The antiferromagnetic or pinning layer comprises a compound from the group of orthoferrites, which show a variety of advantages. For example, these antiferromagnets can have a Nxc3xa9el temperature TN ranging from at least 623 K to 740 K depending on the compounds, and they can display a weak ferromagnetic moment. Therefore, a magnetic device comprising the mentioned structure can be used properly in an environment of a high operating temperature.
Such a compound can be described by the formula RFeO3 with R a rare earth element or Yttrium. The possibility of substitution of the rare earth element shows the advantage that the basic properties, e.g. TN of the antiferromagnetic layer, can be adapted according to application requirements. In this structure, each element can be partially alloyed or substituted by other elements with the same valence such as for instance R1xe2x88x92xR*xFeO3xc2x1y where R* is also a rare earth element and the index x is defined by x xcex5 {0, . . . , 1}. The compound can be also described by the formula RFe1xe2x88x92xTmxO3 with R a rare earth element or Yttrium, and TM a transition metal which can be one element of the groups IB to VIII. This allows a broad variety of possible combinations and an adaption of the pinning layer""s properties according to special requirements.
The compound can be also doped by an element S being an element with another valence such as Barium, Strontium, Calcium, Potassium, or Sodium in compound R1xe2x88x92xSxFeO3. The index x xcex5 {0, . . . , 1}. It is an advantage that the Nxc3xa9el temperature TN of the antiferromagnetic layer can be tuned between about 450 K and 760 K by the use of a suitable dopant. Furthermore, the oxygen stoichiometry can be changed as in RFeO3xc2x1y with y close to zero.
When the antiferromagnetic layer can be made very thin, for instance less than 50 nm, then the advantage occurs that the distance between a magnetic bit on a hard disk and a ferromagnetic layer of a sensor can be reduced. In general, the sensor can be brought much closer to the disc which can be advantageous for the density, the sensitivity, and the operation speed.
If the antiferromagnetic layer can be subjected to an appropriate strain, than the advantage occurs that the properties of the antiferromagnetic layer are tunable. Furthermore, the structure of the antiferromagnetic layer can be distorted which also might change or adapt the properties of the antiferromagnetic layer.
A magnetic device according to the present invention can be part of the a magnetic recording sensor, e.g. a magnetic read head or a GMR sensor, a magnetic random access memory (MRAM), or even a magnetic recording medium. In general, the invention can be used wherever a shifted magnetization loop or an adapted pinning layer is needed.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.